Make sure your difficulty setting is set to at least "normal" and then complete your ranking to 23/23(you may need to beat the game a few times to do this). After you get your last ranking, you will be able to endure all damage except falling into a pit of nails or falling into a hole.
To play as a boss beat the game on the hardest difficulty and let the
credits roll by. When you start your next game the boss will be avalible
